The present invention relates to an optical system having a dual field of view. More particularly, the present invention relates to a simple, space-efficient mechanical actuation of a field of view change function made possible by a cassegrainian configuration. (The precise definition of a Cassegrain system is one consisting of a concave primary mirror which is specifically a parabola and a convex secondary mirror which is specifically a hyperbola. However, systems comprising a concave primary mirror and a convex secondary mirror are now often referred to as cassegrainian systems without particular reference to the particular geometry of the mirrors.)
The present invention provides a dual field of view without the complexity typically found in prior art systems. See, for example, the following U.S. patents:
______________________________________ 2,761,072 Wormser Aug. 28, 1956 2,798,961 Wormser July 9, 1957 2,798,962 Wormser July 9, 1957 3,881,103 Menke et al Apr. 29, 1975 ______________________________________
In the systems disclosed in each of these patents, at least one additional movable mirror or prism has been added for the purpose of providing a dual or multiple function. As explained further below, this is in contrast with the present invention, which employs a simple, space efficient movement of an optical element which is not added to the system but, rather, is an inherent part of a cassegrainian arrangement.